Hint 1: random.seed is called with an unkown, but small value which may be bruteforced.
Hint 2: The random.choices call is taking random characters from the flag to get the string given in the output.txt file. If you know the random seed, you can figure out which random characters it took and reverse the process to unscramble the flag.
Hint 3: Use the flag format (DUCTF{...}) to check if your guess for the seed was correct.